The present invention relates generally to a process for removing copper and other metal contaminants which are present in silicon wafers and, more particularly, to a process for diffusing copper and other metals to the surface of such wafers from which it can be removed while avoiding the formation of copper and other metallic silicide precipitates.
Silicon device performance is degraded by metallic contaminants. Transition metals, including copper, iron and nickel, will dissolve and diffuse during the normal thermal cycles associated with integrated circuit manufacturing conditions. While cooling from these high temperatures, these and other metals may precipitate or concentrate at the surface of the wafer, as well as at interfaces, defect sites, and highly doped regions of the wafer. These precipitates often result in the formation of dislocations and other defects. The precipitates and their associated dislocations and defects, if present in the device fabrication region of the wafer, may render integrated circuits prepared from the wafer useless.
Gettering techniques have been used in an effort to reduce or eliminate the concentration of metallic contaminants in the bulk of a silicon wafer. Such techniques involve the introduction of defect sites into the silicon wafer at locations which do not disturb the functioning of integrated circuits fabricated from the wafer. Gettering techniques, however, are not without limitations. For example, for relatively fast diffusing metals such as copper and nickel, exposure to high fabrication temperatures may enable these metals to escape from gettering sites and reach the device region. Additionally, the defects introduced into silicon as gettering sinks can themselves reduce the quality of silicon, for example, by reducing minority carrier recombination lifetime.
Due to the problems associated with the presence of metallic contaminants, as well as the limitations associated with existing means by which to remove these contaminants, industry has primarily focused its attention on identifying and eliminating, to the extent possible, the sources of these contaminants. Such an approach has become increasingly more costly, however, due to the ever more stringent limitations being placed upon metallic contaminant levels in silicon wafers by integrated circuit manufactures. For example, integrated circuit manufacturers typically require that the concentration of copper contaminants on the surface of silicon wafers be no more than 1xc3x971010 to 1xc3x971011 atoms/cm2. It is foreseeable that this requirement will become approximately 1xc3x97109 atoms/cm2 or less since a large fraction of random device failures can be traced to copper silicide precipitates. Under appropriate conditions, copper reacts with silicon to form a copper silicide precipitate, sometimes referred to as haze defects because, upon being subjected to a common etching treatment and bright light inspection, such defects appear as a haze on the surface of the wafer.
The manufacture of silicon wafers involves a number of steps which are potential sources for the introduction of metallic contaminants, such as wafer polishing. As a result, a need continues to exist for a process which affords both an efficient and effective means by which to remove metallic contaminants from the wafer while preventing the formation of harmful precipitates.
Among the objects of the present invention, therefore, may be noted the provision of a process for removing metallic contaminants from a nonpatterned silicon wafer; and the provision of a process in which metallic contaminants are oxidized, complexed and, as such, removed after being diffused to the surface of a wafer.
Briefly, therefore, the present invention is directed to a process for removing a metallic contaminant from a nonpatterned silicon wafer by immersing the wafer in an acidic solution heated to at least about 125xc2x0 C. for at least about 0.1 hours in order to diffuse the metal to the wafer surface, oxidizing and complexing the diffused metal at the wafer surface, and removing the oxidized metal complex from the wafer surface.
The present invention is further directed to a process for removing a metallic contaminant from a nonpatterned silicon wafer by immersing the wafer in an acidic solution heated to at least about 150xc2x0 C. in order to diffuse the metal to the wafer surface, oxidizing and complexing the diffused metal at the wafer surface, and removing the oxidized metal complex from the wafer surface.
Other objects and features of this invention will be in part apparent and in part pointed out hereinafter.